The present invention relates to an intermediate sleeve which is adapted for use in flexographic or gravure printing systems, and more particularly to a bridge mandrel which is adapted to be mounted onto a printing cylinder and adapted to receive replaceable printing sleeves in flexographic or gravure printing systems.
In a typical flexographic printing process, a flexographic printing plate is attached to a cylinder, and as the cylinder rotates, the inked plate provides an image onto a substrate carried on an impression drum. The art conventionally provides the printing plate in the form of a printing sleeve which is expandable by air pressure for mounting and demounting onto the print cylinder. Typical flexography presses operate at high speeds, sometimes printing over 600 linear feet of paper per minute. These high printing speeds require that the print cylinders and sleeves also rotate at high speeds. The construction of the printing cylinders and printing sleeves can vary, and different constructions have been used to attempt to optimize their printing performance.
As known in the art, the diameter of the inner surface of an air-mounted printing sleeve must be slightly smaller than the diameter of the outer surface of the printing cylinder. The difference in these diameters is a dimension known as the interference fit. Moreover, the diameter of the inner surface of the printing sleeve must be expandable by the provision of pressurized air between the opposed surfaces of the sleeve and the printing cylinder in order to mount such printing sleeves onto the printing cylinders as well as remove the sleeves therefrom.
Typically, a printing job will involve an "image repeat," which is the circumferential length of the text and graphics that are to be printed one or more times on the substrate with each revolution of the printing sleeve. The circumference of the printing sleeve must be large enough to contain at least one image repeat. The sleeve repeat, which is equivalent to the sleeve's circumference (including the printing plate mounted on the sleeve), can contain a number of image repeats. Different printing jobs involve image repeats that differ in size, and different printing jobs require sleeve repeats that differ in size. The larger sleeve repeat sizes require printing sleeves with larger circumferences, which means larger outer diameters. When a "converter," i.e., the operator of the machinery that uses a printing sleeve, orders a printing sleeve that is set up with the printing plates for a job that demands a given sleeve repeat size, the inner diameter of that printing sleeve is determined based on the outer diameter of the printing cylinders on hand in that converter's inventory. This is because the printing sleeve must be mounted on a printing cylinder that has a commensurate outer diameter.
To perform a job that requires a large sleeve repeat size, the diameter of the outer surface of the printing sleeve must be large enough to yield the large sleeve repeat size. This requires printing cylinders with larger outer diameters to support thin printing sleeves. However, new printing cylinders are expensive. As one alternative to incurring this expense, thicker printing sleeves resulting from multiple layers are used instead of the single layer, so-called "thin" sleeves. Thompson et al (U.S. Pat. No. 5,544,584) and Maslin et al (U.S. Pat. No. 4,583,460) describe multi-layer printing sleeves that can be mounted on relatively smaller diameter printing cylinders. Such multi-layer printing sleeves have the effect of reducing the inner diameter of the sleeve so that the sleeve can be mounted on a smaller diameter printing cylinder that is already available in the converter's inventory. Multi-layer sleeves are less expensive than printing cylinders, but more expensive than thin sleeves.
Moreover, it is more costly in labor to change printing cylinders on the printing machinery than it is to change printing sleeves on a printing cylinder. However, this solution has lead to a proliferation of multi-layer printing sleeves, which are more costly than the thin sleeves.
In other sleeve-mounting systems, larger repeat sizes can be printed using a thin sleeve mounted on an intermediate sleeve, also known as a bridge mandrel, that can be provided with pressurized air to mount and dismount the thin printing sleeve. In one such bridge mandrel system, as described in Rossini, U.S. Pat. No. 5,819,657, the mandrel is provided with internal "plumbing" in the form of air inlets, fittings, and passageways so that air may be supplied to its outer surface. One major disadvantage of this type of bridge mandrel construction is that it must have a relatively thick wall to accommodate the "plumbing." This makes the bridge mandrel relatively heavy as well as increasing its cost to manufacture. Nelson, U.S. Pat. No. 5,904,095, also describes a similar mandrel construction which includes internal air passages.
Another type of bridge mandrel simply provides a relatively thin spacer sleeve open at both ends and equipped with air holes such as the sleeve described in Rossini, U.S. Pat. No. 5,782,181. However, in order for pressurized air to be supplied, the mandrel must be fitted with plugs at either end to seal those ends, or, the air hole pattern on the mandrel must be carefully aligned with the air hole pattern on an underlying print cylinder. However, as there are no standard air hole patterns in the art, it becomes problematic to achieve proper air hole alignment in all cases.
Accordingly, there remains a need in this art for a bridge mandrel construction which is simple to manufacture, light weight, and easy to mount and dismount from underlying printing cylinders in flexographic and gravure printing systems.